The present invention relates to electrical antennas and particularly to an improved backfire bifilar helical antenna.
The backfire bifilar helical antenna is a circularly polarized antenna having two opposed helical wires fed at one end with balanced currents. When operated above the cutoff frequency of the helical waveguide, the bifilar helix produces a beam directed along the structure toward the feed point. The term "backfire" is used to describe this direction of radiation in contrast with "endfire" which denotes radiation away from the feedpoint. A detailed study of the antenna appears in a report by Willard T. Patton dated September, 1962 which is available from the Defense Technical Information Center (DTIC), Alexandria VA under catalog number AD 289084.
An example of a backfire bifilar helical antenna is shown in U.S. Pat. No. 4,014,028 to John A. Cone et al. The antenna consists of two interlaced helixes fed at one end. Each helix is fed with energy of equal amplitude and 180.degree. out of phase by a transmission line comprising, for example, a double quarterwavelength slot balun. Optionally the helixes may be connected to a ground plane at the other end for mechanical stability and an insulating right cylinder may be used to support the helixes. The two helixes have a diameter and a pitch to provide a backward wave structure. The direction of energy flow is away from the feed end but the direction of phase progression is toward the feed end.
The bifilar helix is relatively simple when compared to other types of antennas with similar performance. It has an advantage over the conventional axial mode (single element) helix in that the two elements are fed in a balanced mode and do not require a ground plane. The axial mode helix is useful in applications where narrow beamwidth, moderate gain is needed. The backfire bifilar helix can be used in these situations, and in addition, by adjusting the geometry of the helix, it can also be made to supply a wide beam with lower gain for such applications as satellites and ground stations.
A helical beam antenna having a uniform conductor has a nominal impedance which is not convenient for certain applications. John D. Kraus reported in IEEE Transactions on Antennas and Propagation, VOL AP-25, No. 6, November, 1977, pg. 913, that the nominal impedance of an axial fed helix (single element) of uniform conductor size may be adjusted by increasing the conductor size close to the feed point at the ground plane. This lowers the characteristic impedance of the conductor-ground plane combination (acting as a transmission line) and transforms the helix impedance to a lower value over a substantial bandwidth. The teaching of Kraus is directed to an axial mode helix fed against a ground plane and requires that the spacing between the conductor and the ground plane be adjusted to achieve the desired impedance.
In the case of the bifilar helix, two elements are fed in a balanced mode and no ground plane is required. In view of the potential disadvantage caused by poor input impedance, it would be desirable to provide a backfire bifilar helix antenna capable of providing a good impedance match over a broad band of operating frequencies. What would especially be desirable is to design a backfire bifilar helix antenna having a predetermined input impedance which could be manufactured on a production basis without need for adjustment and costly individual fine tuning. In addition it would also be desirable to provide a backfire bifilar helix antenna having a simplified feed arrangement that avoids the complexities of conventional folded and split shield baluns.